In the manufacture of printed wiring boards (PWB) it is customary to coat the PWB with a polymer coating at all locations except where electrical contact is to be made with the metal conductors. This polymer coating is referred to as solder mask or solder resist. When the solder-masked PWB is populated with components and wave soldered, the solder mask confines the solder to the conductors and prevents short circuits between adjacent conductors. The solder mask remains on the PWB and serves as a life-long insulation and environmental barrier.
In common use are three major types of solder mask polymers: thermal curing expoxy, ultra-violet (UV)-curing photopolymers and dry film photopolymers. The epoxies and UV-curables are applied to the PWB as liquid coatings screen printed, while the dry film solder mask is applied in a vacuum chamber.
This disclosure shows a method for achieving an improved solder mask with the available dry film photopolymers, UV-curables, and thermal-curing epoxies.
Dry film solder mask photopolymers are supplied by two manufacturers, in roll form, consisting of an inner layer of photopolymer sandwiched between a carrier film of clear polyester and a polyolefin liner. On a typical PWB the metal conductors extend 0.003 to 0.004 inch above the base laminate, a fact which complicates the application of the dry film solder mask onto the PWB surface. The use of a roller laminator is not usually satisfactory, as air is trapped between the photopolymer and the PWB laminate, particularly between closely-spaced conductors. A vacuum chamber laminator is normally used to prevent air entrapment. The lamination cycle is as follows: A PWB and a section of dry film solder mask is inserted into the heated chamber; the chamber is evacuated, and when up to temperature the photopolymer is forced into contact with the PWB, effecting an air-free lamination. Lamination occurs at a temperature of the order of 200 degrees F., at which temperature the dry film solder mask photopolymer becomes tacky and adheres to the PWB surface. The solder mask is completed by exposing the photopolymer to a strong UV light source through a photographic film, wherein the light hardens the exposed photopolymer. The polyester carrier film is then peeled away and the unhardened photopolymer is washed out in a solvent spray bath.
There are several shortcomings with the dry film solder mask which this invention helps to overcome. These are listed below:
1. The cost of the achieved solder mask is excessive. The single photopolymer layer is expensive being of the order of four times the cost of liquid photopolymers; the equipment required to laminate is overly complicated and expensive; the labor required is excessive, as each processing step of laminating, exposing, and developing is overly lengthy.
2. The laminating step forces photopolymer into circuit holes, with the result that with small holes of the order of 0.025 inch diameter, the photopolymer does not wash out, leaving the holes plugged.
3. The laminating step forces photopolymer into large tooling holes and slots, leaving a puckered, striated coating of photopolymer, for the polyester carrier film is non-conforming and wrinkles around larger holes and sharp corners.
4. The dry photopolymer is characterized by a lack of adhesion to metal conductors. For PWB having bare copper conductors with dry film solder mask thereover, the solder coating and hot-air leveling step is usually not satisfactory without copper surface pretreatment as with a black oxide coating; otherwise the solder mask separates from the conductors.
5. The photopolymer is temperature sensitive, delaying the application sequence. After laminating at 200 degrees F., the photopolymer must cool to room temperature prior to exposure. After exposure, which raises the temperature again, the photopolymer must cool to room temperature before washing out unexposed photopolymer.